1. Field of the Invention
This invention relates to an FM stereo signal demodulating apparatus and method for demodulating an (Lxe2x88x92R) signal multiplexed on an (L+R) signal included in an FM detected signal to acquire an L signal and an R signal.
2. Description of the Related Art
In the reception of FM stereo broadcasting, when a received FM wave is FM detected, as seen from FIG. 6A, an (L xe2x88x92R) signal has been multiplexed on an (L+R) signal. In order to demodulate such a multiplexed signal to an L signal and an R signal, an FM stereo signal demodulating apparatus as shown in FIG. 4 has been proposed.
In FIG. 4, the FM stereo signal demodulating apparatus includes a 19 KHz bandpass filter (BPF) 1 for extracting a signal at 19 KHz from the FM detected signal, an approximated sine wave generator 30 for generating a carrier signal at 38 KHz from the signal at 19 KHz, a multiplier 31 for multiplying the FM detected signal by the carrier signal at 38 KHz to demodulate the (Lxe2x88x92R) signal, an adder 7 and a subtracter 8.
The carrier signal at 38 KHz and the FM detected signal are multiplied so that the (Lxe2x88x92R) signal is demodulated as shown in FIG. 6B.
The signals shown in FIGS. 6A and 6B are subjected to addition in the adder 7 to produce an L signal of 2L. The signals shown in FIGS. 6A and 6B are also subjected to subtraction in the subtracter 8 to produce an R signal of 2R.
The L signal and the R signal are supplied to a speaker (not shown) in which these signals are converted into an audio signal.
The other multiplexed signal than 2L is also produced from the adder 7 and the other multiplexed signal than 2R is also produced from the subtracter 8. However, these multiplexed signals are outside an audio frequency range and hence are not produced as sound from the speaker.
The FM stereo signal demodulating apparatus explained with reference to FIG. 4 is operated through digital signal processing and the approximated sine wave generator 30 the details of which is shown in FIG. 5 generates a signal at 38 KHz.
In FIG. 5, the approximated sine wave generator 30 includes a 90-degree phase shifter 21 for 90-degree phase shifting the phase of the 19 KHz signal, an XOR 22 for taking an exclusive OR between the 19 Hz signal and the output signal from the 90-degree phase shifter 21, another 90-degree phase shifter 23 for 90-degree shifting the phase of the output signal from the XOR 22, another XOR 24 for taking an exclusive OR between the output from the XOR 23 and the output from the 90-degree phase shifter 23, an inverter 25, still another 90-degree phase shifter 26 for 90-degree phase shifting the output signal from the inverter 25, still another XOR 27 for taking an exclusive OR between the outputs from XOR 22 and from the 90-degree phase shifter 90, a multiplier 32 for multiplying the output signal from the XOR 27 by {square root over (2)}xe2x88x921, and an adder 33 for adding the output from the XOR 22 and the output from the multiplier 32.
An explanation will be given of the operation of the approximated sine wave generator 30.
When the 19 KHz signal shown in FIG. 3A is fed to the approximated sine wave generator 30, the signal shown in FIG. 3B is produced from the 90-degree phase shifter 21, and the signal shown in FIG. 3C is produced from the XOR 22.
The signal shown in FIG. 3D is produced from the 90-degree phase shifter 23, and the signal shown in FIG. 3E is produced from the XOR 24.
When the output from the XOR 24 is inverted by the inverter 25 and 90-degree phase-shifted by the 90-degree phase shifter 26, the signal shown in FIG. 3F is produced from the 90-degree phase shifter 26.
The exclusive OR between the output from the XOR 22 and the output from the 90-degree phase shifter is taken by the XOR 27 so that the signal shown in FIG. 3G is produced from the XOR 27.
The output from the XOR 27 is multiplied by {square root over (2)}xe2x88x921 in the multiplier 32, the signal having an amplitude of 0.414 indicated by dotted line in FIG. 3H is produced from the multiplier 31.
The output from the XOR 22 and output from the multiplier 32 are added in the adder 33 so that the signal shown in FIG. 3I is produced from the adder 33.
The signal shown in FIG. 3I has a period of 38 KHz that is twice as long as that of the 19 KHz signal and an amplitude approximated to a sine wave.
As described above, in the above conventional FM stereo signal demodulating apparatus, as shown in FIG. 4, the multiplier 31 multiplies the m bit signal from the approximated sine wave generator 30 and the FM-detected n bit signal.
The multiplication of m bitsxc3x97n bits is carried out by adding m bits by n-times. This takes a relatively long time. In order to complete the multiplication within a prescribed time, the multiplier which can be operated at a high speed is required. This leads to an increase in the production cost.
An object of this invention is to provide an FM stereo signal demodulating apparatus and method which can be operated at a low speed.
In order to attain the above object, in accordance with a first aspect of this invention, there is provided an FM stereo signal demodulating apparatus comprising:
a filter for extracting a 19 Hz signal from an FM detected signal;
a first phase-shifter for phase-shifting the 19 Hz signal by 90 degree;
first logic circuits for taking an exclusive OR between an output from the first phase-shifter and the 19 KHz signal;
a second phase-shifter for phase-shifting by 90 degree an output signal from the first logic means;
second logic circuits for taking an exclusive OR between an output from the first logic means and an output from the second logic means;
an inverter for inverting an output from the second logic means;
a third phase-shifter for phase-shifting an output signal from the inverting means;
third logic circuits for taking an exclusive OR between an output from the third phase-shifter and an output from the first logic circuits;
a first multiplier for multiplying an output from the first logic circuit and the FM detected signal;
a second multiplier for multiplying an output from the third logic circuit and the FM detected signal;
a third multiplier for multiplying an output from the second multiplier by {square root over (2)}xe2x88x921;
a first adder for adding an output from the first multiplying means and an output from the third multiplier;
a second adder for adding the FM detected signal and an output from the first adder; and
a subtracter for subtracting an output from the first adder from the FM detected signal.
Preferably, the third multiplier makes multiplication by 2xe2x88x921 instead of that by {square root over (2)}xe2x88x921.
Preferably, the first multiplier is adapted so that if the output from the first logic circuits is xe2x80x9c1xe2x80x9d, the FM detected signal is passed, whereas if the output from the first logic circuits is xe2x80x9c0xe2x80x9d, the FM detected signal is inverted, or otherwise if the output from the first logic circuits is xe2x80x9c0xe2x80x9d, the FM detected signal is passed, whereas if the output from the first logic circuits is xe2x80x9c1xe2x80x9d, the FM detected signal is inverted.
Preferably, the second multiplier is adapted so that if the output from the third logic means is xe2x80x9c1xe2x80x9d, the FM detected signal is passed, whereas if the output from the first logic circuit is xe2x80x9c0xe2x80x9d, the FM detected signal is inverted, or otherwise if the output from the third logic circuit is xe2x80x9c0xe2x80x9d, the FM detected signal is passed, whereas if the output from the third logic means is xe2x80x9c1xe2x80x9d, the FM detected signal is inverted.
In accordance with a second aspect of this invention, there is also provided a method of demodulating an FM stereo signal using the apparatus according to the first aspect.
In accordance with this invention, the 2 bits produced from the approximated sine wave source and the FM detected signal are individually multiplied and thereafter the multiplied results are added. Therefore, the multiplication is carried out as multiplication by 1 bit so that the multiplexed signal can be easily demodulated.
The above and other objects and features of the invention will be more apparent from the following description taken in conjunction with the accompanying drawings.